I. Field of the Invention
The present invention is concerned with exchangeable filter elements for use in nuclear installations for the purification of airstreams or gas streams which contain toxic or radioactive dust. The invention is furthermore concerned with a method for the exchange and disposal of the filter elements.
II. Description of the Prior Art
In existing nuclear installations the operating air must be purified and materials which are dangerous to health, such as toxic or radioactive matter, must be separated therefrom. This is accomplished in special filter installations which are equipped with filter elements which remove suspended matter that is dangerous to health. A special problem with regards to environmental protection is the necessary exchange of the contaminated elements and their disposal. This must be accomplished without creating the danger of contamination especially for the personnel who are concerned with the exchange.
The filter elements for suspended matter which were used for such purposes usually consisted of a filter layer for suspended matter which was formed by the folding of suitable paper in a zig-zag shape. This filter layer is mounted in a rectangular- or square-shaped frame and sealed with a sealant.
These filter elements which consist of a frame and the filter medium are, for instance, inserted into a housing which is connected to an air duct where the circumferential edge of the rigid filter element frame is kept in tight contact by means of tightening devices with a tightening area located inside the housing, in such a way that the air which should be purified must necessarily pass the filter layer without bypass. In this case the filter element is introduced from the side, through an opening which is located in the housing, and which opening can be closed air tight by means of a door or a lid which can be flipped down. The exchange of the contaminated filter element is carried out by employing the so-called "protective bag technique". In this arrangement the filter element to be exchanged is pulled into a bag, which bag is connected to the housing and where the filter element is sealed in by sealing off the protective bag. For the introduction of a new filter element, a second protective bag is attached at the housing and the remainder of the first protective bag is pulled into the second protective bag, whereupon the new filter element can be introduced into the corresponding housing. The final disposal of the contaminated filter elements, which are sealed in the protective bag, is very expensive and time consuming because of the existing danger for the operating personnel and for the environment. The compaction of the filter elements is carried out employing impact mills, shredder installations and saws. Compacting presses are used to achieve an appropriate compacting. These operational steps are carried out in a protective atmosphere as distinguished from the breathing air. This is also necessary for the further handling and control of these machines. The remainder of the filter elements, which are very contaminated and which were treated in this way, must then be put into standardized, drum-shaped waste containers, which waste containers are then stored in inaccessable or protected places, such as subterranean storage places.
Since the square-shaped known standardized filter elements do not fit into the waste containers or only partially fill these waste containers, they must be compacted, using the protective devices which were described above before they are put into these containers. Compacting is necessary in order to achieve optimum, economic utilization of the waste container volume for the purpose of cost reduction. After the compaction has been achieved the remainder of the contaminated filter elements are put into the drum-shaped waste container until the container is approximately filled. Then the waste containers are closed air tight and are transported to the final storage areas.
In order to reduce the danger of damage to the protective bag by means of the rigid filter element frame and especially by means of the corners and edges of the rigid element frame, a rectangular or square filter cell with rounded edges is disclosed (West German Patent No. DBGM 6 608 707) where the filter element, which consists of filter paper which is folded in a zig-zag shape, is located in an inner frame. The frame may be reused. The filter is in a frame at the air entrance and at the air exit, tightly connected with the outer frame by means of a plastic seal and by means of an adhesive tape which is equipped with a tear string. In case of an exchange of the filter element, the outer frame with the filter element must be received in a sealed, protective bag, whereupon in the protective bag the filter paper element is released from the rigid outer frame by pulling off at the tear. When inside the protective bag, the filter element must be pulled out of the outer frame and the protective bag must be sealed off and separated from the outer frame. In this way the rigid outer frame, after it has been decontaminated, can again be used and can be equipped with a fresh filter element; however, the necessary decontamination of the outer frame is rather time consuming. In addition, even this compressible filter element must be compressed by means of breaking, cutting or sawing or pressing before it is put into the waste container, since it cannot be compressed in a simple manner. This necessitates, at first, storage of the filter elements in their safe conditions with the appropriate expense prior to the necessary compaction work and disposal work. For this filter element the treatment, which has been described above, is complicated, expensive and time consuming and necessitates extensive safety measures.
III. Prior Art Statement
In the opinion of the applicant, the above-mentioned prior art represents the most pertinent and relevant prior art of which applicant is aware.